2 Answers
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Suppose you have a single wire and you connect it to a battery. Electrons start to flow, but as they do so the resistance to their flow (i.e. the resistance of the wire) generates a potential difference. The electron flow rate, i.e. the current, builds up until the potential difference is equal to the battery voltage, and at that point the current becomes constant. All this happens at about the speed of light.

Now take your example of having let's say two wires (A and B) with different resistances connected between the wires - lets say $R_A \gt R_B$. The first few electrons to flow will be randomly distributed between the two wires, A and B, but because wire A has a greater resistance the potential difference along it will build up faster. The electrons feel this potential difference so fewer electrons will flow through A and more electrons will flow through wire B. In turn the potential along wire B will build up and eventually the potential difference along both wires will be equal to the battery. As above this happens extremely rapidly.

So the electrons don't know in advance what path has the least resistance, and indeed the first few electrons to flow will choose random paths. However once the current has stabilised electron flow is restricted by the electron flowing ahead, and these are restricted by the resistance of the paths.

To make an analogy, imagine there are two doors leading out of a theatre, one small door and one big door. The first person to leave after the show will pick a door at random, but as the queues build up more people will pick the larger door because the queue moves faster.

They don't. Electrons follow the path of least resistance in the same way that water flows downhill. The electrons do not act collectively, each individual electron is driven away from other electrons, and driven toward positive charges. The collective result is well described by the statement that they follow the path of least resistence.

Suppose water flows down a hill in two paths that look very same from the top, but one of them has a dam built on the way. Won't water flow through that path? But water flowing down the other path is surely happier. How do you explain ths?
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Swapnanil SahaAug 7 '12 at 12:38

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Water flows down both paths. When it reaches the dam, it fills it. As long as the water flowing into the dam is greater than that flowing out of it, the level will rise. No, the water isn't "happier" (I think this is a translation issue). The water goes where the water goes and it goes that way because of the forces acting on it; the same is true of the electricity.
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AdamRedwineAug 7 '12 at 12:55

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@SwapnanilSaha Water doesn't flow down the hill just because the bottom of it is lower, it flows down because the hill is sloped at the position of the water molecules.
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DarthfettAug 7 '12 at 15:30

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I think this is kind of a bad analogy, unless you imagine the water to be flowing inside pipes. A blockage in a pipe will cause higher pressure further up, and that's why the water doesn't flow into it.
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NathanielAug 7 '12 at 15:47

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@AdamRedwine: OK. I just meant that water on ground would be at lower potential than water up on the hill, and so, "happier".That is what everything wants, and if one gets what one wants, one is "happy".That's all.
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Swapnanil SahaAug 7 '12 at 17:42